Electric discharge apparatus



y 1958 Etc. HARTWIG 2,834,918

ELECTRIC DISCHARGE APPARATUS Filed Sept. 4, 1955 s Sheets-Sheet 1 Fig.l.

' I 2 El 1-! POWER SUPPLY w UNIT v E2 I WELDER SEQUENCE TIMER ALB Tim 9- a n ga/ I ATTORN EY E. C. HARTWIG ELECTRIC DISCHARGE APPARATUS May13, 1958 3 Sheets-Sheet 2 Filed Sept. 4, 1953 wmmgu mm wnnssszs 4 v mvsmon Edward C.Hortwig. 73 95;

ATTORNEY y 1958 E. c. HARTWIG 2,834,918

ELECTRIC DISCHARGE APPARATUS Filed Sept. 4, 1953 3 Sheets-Sheet 3 ALI WITNESSES: INVE'NTOR Edward C.Horfwjg.

ATTORNEY United States Patent ELECTRIC DISCHARGE APPARATUS Edward C. Hartwig, Walnut Creek, Calif., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 4, 1953, Serial No. 378,546

11 Claims. (Cl. 315-230) is costly. Because the discharge devices and other comice succeeding device through a timing network. The anode circuit of each device is supplied through inductive reactance means and when a device is rendered conducting during any half period of the supply it continues to conduct because of the inductive reactance in its anode circuit for a time interval longer than the half period and becomes non-conducting after its anode potential has become negative and the anode potential of the succeeding device has become positive. This phenomenon may be called the carry-over effect.

For convenience the relationship of two devices which are rendered conducting in succession to time one of the operations may be here considered. The first device to conduct may be called the leading device and the second device to conduct may be called the following device. The leading device is so connected that before it becomes non-conducting it impresses its arc drop potential, as a firing potential in the control circuit of the following device at a time when the anode potential of the latter has just become positive. The following device is then rendered conducting substantially at the beginning of its ponents have a tendency to deteriorate as they age this I apparatus has a comparatively high maintenance cost.

It is accordingly an object of my invention to provide a sequence timer of lower initial and maintenance costs than that disclosed in my above-mentioned patent.

A general object of my invention is to provide a sequence timer of relatively simple and low cost structure which shall have a low maintenance cost.

An incidental object of my invention is to provide highly precise timing apparatus.

Another incidental object of my invention is to provide timing apparatus which shall be adapted to be supplied from an alternating current source and which shall operate precisely in synchronism with the source.

To help in the understanding of my invention, a brief discussion of the circuit disclosed in my earlier patent appears desirable. This circuit includes a number of main discharge devices, specifically thyratrons, each of which is rendered conducting to initiate each of the timing periods of the timed process. But the main thyratrons are not rendered conducting directly in sequence. Between successive main thyratron stages a transitional stage is included and this latter stage includes an auxiliary thyratron. There are thus at least two as many thyratrons as there are operations in a process.

My invention arises from the realization that the circuit disclosed in my patent would be considerably simplified and its cost would be reduced if the transitional discharge devices and their components were eliminated, and in accordance with the broad aspects of my invention, I provide such a circuit. Specifically, in accordance with my invention, I provide a plurality of discharge devices, one for each of the operations of a process. The discharge devices are rendered conducting in succession to initiate each of the timed operations. To accomplish this object without transitional stages, the discharge devices are supplied from three conductors, one of which is at an intermediate potential relative to the others. Specifically the potentials on the latter conductors are in opposite phase and the intermediate potential conductor may be regarded as a neutral. The anode potentials of devices which initiate succeeding operations are derived alternately from opposite phase conductors and are of substantially opposite phase. Each device controls the timing of a succeeding device, the anode circuit of each device being connected to the control electrode of the positive half period of the supply. The operation of each following device is thus controlled precisely in synchronism with the supply.

In the practice of my invention, discharge devices of any type which manifest the carry-over effect or even transistors which manifest this effect may be utilized and the expression electric discharge device herein is intended to mean any such discharge device or transistor. But I prefer to use thyratrons and particularly low cost thyratrons such as the WL2050. I have found that such a thyratron has a tendency to become inoperative if a negative potential of substantial magnitude is repeatedly, over an interval of-several hours, impressed between its control electrode and the cathode, while the device is conducting. It is my understanding that this inoperativeness is caused because the gas ions in the thyratron are attracted by the high negative potential on the control electrode and are imbedded in the control electrode, that is the thyratron cleans up. In circuits of the abovedescribed type each leading discharge device is so connected in the control circuit of a following discharge device that once the leading discharge device becomes non-conducting, the negative potential which abruptly appears at the anode of this leading device is impressed between the control electrode and the cathode of the following device. Since at this time the following device is already conducting, the repeated impressing of this negative potential if it is of substantialsmagnitude would have the above-described tendency to render the following device inoperative.

It is accordingly a specific object of my invention to provide a circuit of the above-described type in which clean up of the thyratrons shall be suppressed.

A more specific object of my invention is to provide means in a circuit of the above-described type for reducing the negative potential impressed between the control electrode and the cathode of a following device when a leading device abruptly becomes non-conducting.

An incidential object of my invention is to provide novel electronic circuits.

In accordance with the specific aspects of my invention I provide a system of the above-described type which includes a network consisting of an impedance and a rectifier means connected in series between that supply conductor and the neutral supply conductor to which the anode of a leading discharge device is connected. The rectifier means is connected to conduct positive current from the neutral conductor to the other conductor, By positive current I means the flow of positive ions or holes as distinguished from electrons. The anode of The conductors ALI and AL3 are thus in the leading device is not connected directly to the supply conductor but to the junction of the impedance and the rectifier. This junction is connected to the control electrode of the following device. The rectifier is poled opposite to the leading device and during the negative half periods of anode-cathode potential for the leading device the potential at the junction is reduced to the small potential drop across the rectifier. This potential is so small as not to produce the clean-up described above.

The novel features that I consider characteristic of my invention are set forth generally above. The invention itself, both as to its organization and method of operation together with additional objects and advantages thereof, will be understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Figure 1 is a circuit diagram of a preferred embodiment of my invention;

Fig. 2 is a graph illustrating the operation of the apparatus shown in Fig. 1;

Fig. 3 is a circuit diagram of a modification of my invention; and

Fig. 4 is a circuit diagram of a further modification of my invention.

Descriptin.-Figure 1 My invention is applicable to industrial processes involving a number of successive operations of different types. In Figure 1 it is illustrated as specifically applied to resistance welding and shown combined in a resistance welding system. In its simplest form, the production of a resistance weld involves at least four timed operations; there is the squeeze interval during which the welding electrodes are engaged with the work and pressure is applied, there is the weld interval during which the welding current flows, there is the hold interval during which the electrodes are held in engagement with the work until the weld nugget button solidifies, and there is the off interval during which the work is reset for a succeeding Weld. The apparatus disclosed in Fig. 1 is conceived to carry out the above-described operations in producing a spot weld.

This apparatus includes a Welder, a Power Supply Unit, and a Sequence Timer. Power for the apparatus is derived from main power supply buses or conductors LI and L2 which may be energized from a commercial alternating current source. Auxiliary supply conductors or buses ALI, AL2, and AL3 are energized from the main buses through transformer T1 which has a secondary S1 having an intermediate tap. The conductors ALI and AL3 are connected to the end terminals of second ary S1 and the conductor AL2 to the intermediate tap. opposite phase with conductor AL2 neutral.

The Welder includes a welding transformer T having a primary P and a secondary S and welding electrodes EI and E2 connected across the secondary S. During a welding process the work W is disposed on one of the electrodes E2 and the electrode E1 is urged into engagement with the work W by means of a fluid operated piston Z. The fluid which actuates this piston is con trolled by valve V which is actuated by solenoid O.

The Power supply unit includes a pair of ignitrons I-1 and 1-2, each having an anode 5, a cathode 7, and an ignitor 9. The ignitrons I-1 and 1-2 are connected as a so-called Weld-O-Trol contactor between the conductors LI and L2 in series with the primary P; the anodes 5 and the cathodes 7 are connected in antiparallel between the conductors LI and L2 and the primary P. Between each ignitor 9 and its associated cathode 5, a pair of rectifiers 11, 13, I5 and I7, respectively, preferably of the dry type, are connected in series in such a direction as to conduct positive current from the cathode 7 to the ignitor 9. Thus, the rectifiers II and 13 are so connected to the ignitor 9 of ignitron II for example that positive charges (or holes) can flow from the cathode 7 through the lower rectifier II and from the lower rectifier 11 through the upper rectifier 13 to the ignitor 9 but not in the reverse direction. The junctions of the pairs of rectifiers are adapted to be interconnected by the normally open contact 19 of a weld relay RW which is actuated by the Sequence Timer, and when so connected the ignitrons are rendered conductive during opposite polarity half periods of the supply conductors LI and L2 and conduct alternating current through the primary P.

The Sequence Timer is supplied from conductors ALI, AL2, AL3. It includes :a plurality of thyratrons which are rendered conductive in succession and which may be designated the oil thyratron OT, the squeeze thyratron ST, the weld thyratron WT, and the hold thyratron HT. In addition, there is an auxiliary thyratron AT. Each of the thyratrons OT, WT, HT, and AT has an anode 21, a cathode 23, and a control electrode or grid 25. The squeeze thyratron ST has an anode 27, a cathode 29, a first control electrode 31, and a second control electrode 33.

In producing a weld the off thyratron OT is rendered conducting first; then the squeeze thyratron ST and auxiliary thyratron AT are rendered conducting; next, the weld thyratron WT conducts and finally the hold thyratron HT conducts. The thyratrons which conduct in succession in this sequence are supplied from opposite phase conductors ALI and AL3, respectively. Accordingly, thyratrons OT and WT are supplied from conductors ALI and AL2; thyratrons ST, AT and HT are supplied from the other conductor AL3 and AL2.

The anode 21 of thyratron OT is connected to conductor ALI through the exciting coil of a starting relay RS and a rectifier 35 so poled as to conduct positive current from the conductor ALI to the anode 21. The cathode 23 is adapted to be connected to conductor AL2 through the normally-open contact 37 of the starting relay RS or through a start switch FS for the apparatus, which may be and usually is a foot switch.

The starting relay RS has an additional normally open contact 39 which when closed connects the solenoid 0 between the conductors LI and L2 to energize the solenoid and open the valve V. Under such circumstances, fluid is supplied over the piston Z and the electrode E1 is brought into engagement with the work W.

Between the conductors ALI and AL2 an impedance R1 is connected in series with a rectifier X1 which may be of the dry type; specifically, the impedance R1 is a resistor. The rectifier X1 is so connected as to conduct positive current from the conductor AL2 to the conductor ALI; that is, opposite to the thyratron OT. Thus, during the half period when the anode-cathode potential on thyratron OT is negative, the rectifier X1 conducts current through the resistor R1 and the drop at the junction 11 between the rectifier XI and the resistor R1 is the small potential drop across the rectifier XI.

The anode 21 of the thyratron OT is adapted to be connected to the junction J1 through a pressure switch SP which is actuated when adequate pressure is built up on the work W. The switch SP thus permits the sequence to proceed only if the pressure on the Work W is adequate. Since the rectifier 35 in series with the coil of the relay RS is so poled as to conduct positive current from the conductor ALI to the anode of thyratron OT, it permits current to flow through the coil RS and the thyratron OT but not through the resistor R1 and the coil RS after the switch SP is closed.

The anode 21 of the thyratron WT is connected to the conductor ALI through the primary A'PI of an auxiliary transformer ATI and through a rectifier 41 poled to conduct positive current from the conductor ALI to the anode 21 of the thyratron WT.

The cathode 23 of the thyratron WT is adapted to be connected to the conductor AL2 through the normally R2 and another rectifier X2 are connected in series.

5 open contact 37 of relay RS and also through the starting switch F S.

Between the conductors'A l and AL2 another resistor This rectifier X2 is also connected to conduct positive current from the conductor AL2 to the conductor AL1; that is, opposite to the thyratron WT. The junction J2 of the resistor R2 and the rectifier X2 is connected to the anode 21 of the thyratron WT.

The anode 27 of the thyratron ST is connected to the conductor AL3 through the exciting coil of the weld relay RW. The cathode 29 is directly connected to the conductor AL2, The anode 2]. of the thyratron AT is connected to the conductor AL3 through a reactor 43 and through a rectifier 45 poled to conduct positive current from the conductor AL3 to the anode. The cathode 23 of thyratron AT is connected directly to the conductor ALZ. Another resistor R3 and rectifier X3 are connected in series between the conductors AL3 and AL2, the recti fier X3 being connected to conduct current from the conductor AL2 to the conductor ALB. The junction J3 of the rectifier X3 and resistor R3 is connected to the anode of the thyratron AT.

The anode 21 of the thyratron HT is connected to the conductor AL3 through the primary AP2 of an auxiliary transformer AT2 and a rectifier 5-7 poled to conduct positive current from the conductor AL3 to the anode. The cathode 23 of the thyratron HT is connected directly to the conductor AL2. The anode of thyratron HT is adapted to be connected to the junction J3 through a contact 49 of a repeat-non-repeat switch RNR when the latter is in the non-repeat position and through a rectifier 51 poled to conduct positive current from the junction J 3 to the anode 21.

In addition to the timing thyratrons OT, ST, WT, HT, and the auxiliary thyratron AT and their anode circuits, the sequence timer includes a plurality of timing networks for timing the various operations of the process. These include an off network ON, a squeeze network SN, a weld network WN and a hold network HN. Each network includes a capacitor 53, 55, 57, 59, and a fixed resistor and a variable resistor 63, 65, 67, 69, 73, 75, 77, 79 in parallel with the capacitor. The variable resistors 73 through 79 may be set to time the discharge of the capacitors 53 through 5% when they are charged and thus to time the operations of the process. The capacitors and the resistors have adequate magnitudes to produce timing of from one-half period of the supply to a large number of half periods as high as or 60 or even higher. The variable resistor 73 of the oif network ON may be shunted out by a contact 81 of the repeat-non-repeat switch RNR when the latter is in the non-repeat position.

In addition to the networks ON, SN, WN and HN there is an auxiliary network AN. This network includes a capacitor 83 shunted by a resistor 85. Preferably the resistor is fixed and is of a magnitude such as to discharge the capacitor 83 in a short time of the order of: a half period of the supply. A system in which the resistor 85 is variable and discharges the capacitor 83 in a long time interval is within the scope of my invention.

The squeeze network SN is connected between the junction 11 and the control electrode 25 of the auxiliary thyratron AT and the first control electrode of the thyratron ST through grid resistors 87 and 69 respectively. When the potential of the conductor ALT is positive relative to the potential of conductor AL2, the capacitor of network SN may be charged by current flowing between the control electrodes 25 and 31 and the cathodes 23 and 29 of thyratrons AT and ST. The weld network WN is connected between the junction I3 and the control electrode 25 of thyratron WT through grid resistor 91. When the conductor AL3 is positive relative to conductor AL2 the capacitor 57 of the weld network negative and the other plate positive.

6 WNrnay be charged by the flow of current between the control electrode 25 and the cathode 23 of the thyratron WT. The hold network Pit-I is connected between the junction J2 and the control electrode 25 of thyratron HT through a grid resistor 93 and its capacitor 59 ischarged similarly to the capacitors of the other networks. The off network ON is connected between the control electrode 25 andthe cathode 23 of thyratron OT through the secondary AS2 of transformer AT2 and a grid resistor 95'. The capacitor of the off network ON may thus be charged when thyratron HT is conductive. The secondary A632 is so connected that whenthyratron HT does conduct, the capacitor ischarged with its plate electrically nearest the control electrode 25 of the thyratron OT negative and the other plate positive. The conduction of thyratron HT thus tends to render thyratron OT nonconductive.

The network AN is connected between the second control electrode 33 and the cathode 29 of thyratron ST through the secondary ASl of the transformer AT1 and a grid resistor 97. The secondary A81 is so connected that when thyratron WT is conductive the capacitor 83 of the network AN is charged so that its plate electrically nearest the control electrode 83 of the thyratron ST is Conduction of thyratron WT accordingly renders thyratron ST non-conductive.

In addition to the components above discussed, there are surge suppressing capacitors 99, 101, 103, 105, 107, 109 which are connected between the control electrodes 25, 31, 33 and the cathodes 23, 29 of each of the thyratrons. The purpose and function of these capacitors are known in the art and need not be discussed at this point.

In the anode circuits of each of the thyratrons OT, ST, WT, HT and AT an inductive reactance is connected. This inductive reactance may be the inductance of a component such as the coils of the relays RS and RW or the primaries AP1 and AP2 of the transformers AT1 and AT2 or it may be a lumped inductance such as the inductance 43 as connected in the anode circuit of thyratron AT. Whatever the inductive reactance in the anode circuit of each of the thyratrons, it should be suflicient to cause the thyratron to conduct after its anode-cathode potential becomes negative at least until the anodecathode potential of the thyratron which follows it in the sequence rises to a suificient magnitude to enable the latter to condut. If necessary, inductive reactance, such as a lumped inductance, may, be added Many of the anode circuits in which the inductive reactance is shown in Fig. 1 as a relay coil or a transformer.

Stand-by for Figure 1 In the stand-by condition of the apparatus the power switch or circuit breaker (not shown) has been closed and the apparatus is ready for use at any time. Under such circumstances the contacts of starting relay RS are open, solenoid O is de-energized, the valve V is closed and the electrode E1 is retracted from the electrode E2. The contact of relay RW is open and the ignitrons I-1 and I-2 are not conducting. The anode-cathode circuit of .thyratron OT is open at-the contact of relay RS and the switch PS. This thyratron is then not conducting. The squeeze network SN is connected through junction T1 to conductor ALI and during the half periods when conductor ALI is positive relative to conductor AL2, the

capacitor 55 of this network is charged by current flow between the control electrodes and the cathodes of thyratrons ST and AT. The charge on the capacitor 55 is such as to block thyratrons ST and AT. The anode-cathode circuit of thyratron WT is open at the normally open contact 37 of relay RS and starting switch FS and thyratron WT is not conducting. Since the anode-cathode circuit of thyratron WT is open at the cathode network WN is uncharged. The network HN is connected through junction I2 to conductor ALI and during the half periods when conductor ALI is positive relative to conductor AL2 the capacitor 59 of network HN is charged so as to impress a blocking potential between the control electrode and cathode of thyratron HT. Since thyratron HT 1s not conducting the capacitor 53 of network ON is uncharged and thyratron OT is ready to conduct once its anodecathode circuit is closed. Since thyratron WT is not conducting the capacitor 83 of the auxiliary network AN is uncharged and there is no blocking bias on the other control electrode 33 of thyratron ST.

0perati0n.Figure 1Repeat The operation of the apparatus shown in Fig. l with the switch RNR set for repeat will first be described.

To produce a weld the work W is disposed on electrode E2 and the start switch FS is closed. The closing of the start switch closes the anode circuit through thyratron OT and the latter immediately conducts and current flows through the coil of relay RS. Relay RS now picks up and at its lower contact 37 shunts out the starting switch FS so that the latter may be released. At the upper now closed contact 39 the solenoid O is connected across the conductors L1 and L2 and the valve V is opened. Pressure is now applied to the piston Z and the electrode E1 is engaged with the work W. When the pressure is adequately built up, the pressure switch SP is closed.

The network WN is connected to conductor AL3 through junction J 3 but as has been seen since the cathode circuit of thyratron WT is open at contact 37 and switch FS capacitor 57 cannot be charged. Depending on the polarity of bus ALI at the instant of closure of operating switch FS, thyratron WT may or may not be rendered conducting. If thyratron WT is rendered conducting, it will remain so for not more than one half period of its anode supply. During the succeeding half period of the supply when bus AL3 is positive relative to bus AL2, the capacitor 57 will be charged so as to impress a blocking potential between the control electrode and cathode of thyratron WT to prevent further conduction of thyratron WT. The minimum time constant of hold network HN as determined by resistor 69 and capacitor 59 is such that conduction of thyratron WT for the one half period of the supply is not sufficient to render thyratron HT conducting. In addition, the minimum time constant of squeeze network SN is such that even though firing of thyratron WT for one half period may develop a blocking potential on network AN, the bias on network AN will decay before the bias on network SN once thyratron WT is rendered non-conducting and consequently the instant of conduction of thyratron ST is not dependent in any way upon the half-period conduction of thyratron WT.

The anode-cathode circuit of thyratron OT is now connected, during the half periods during which it is conducting, in parallel with the squeeze network SN and the control-electrode-cathode circuits of the thyratrons ST and AT. The charging current which had been flowing to capacitor 55 during these half periods to maintain it charged is now shunted away from the network SN and the capaictor discharges.

The discharge of the capacitor 55 of the squeeze network SN causes the potential, which is originally impressed between the control electrodes 31 and 25 and the cathodes 29 and 23 of thyratrons ST and AT, to decay. On this decaying potential the potentials appearing at junction J1 during the opposite half periods of the conductor AL1 and AL2 are superimposed. During the half periods when thyratron OT is conducting this superimposed potential is equal to the arc drop across the thyratron OT which may be of the order of 10 or 15 volts. Since thyratron OT continues to conduct after the potential between conductors ALI and AL2 reverses, the superimposed arc drop potential hangs on after conductor AL1 becomes negative relative to conductor AL2; that is after conductor AL3 becomes positive relative to conductor AL2 and thyratrons ST and AT can become conducting.

8 During the half periods when conductor ALI is negative relative to conductor AL2 the superimposed potential is the small potential drop across the rectifier X1 connected to junction J1.

At the end of the squeeze interval the potential impressed by the squeeze network between the control electrodes 31 and 25 and cathodes 29 and 23 of thyratrons ST and AT has decayed to so small a negative magnitude that the arc drop potential of the thyratron OT becomes effective This potential is or course only effective during ihe =-ovsr portion of the conduction of thyratron OT, that is the beginning of the first half period during which conductor AL3 becomes positive relative to conductor AL2 following the decay of the potential of networks SN to the small magnitude and at this point thyratrons ST and AT are rendered conducting. Thyratrons ST and AT are thus rendered conducting in synchronism with the supply.

The conduction of thyratron ST supplied current to the coil of the relay RW to energize the latter. The normally open contact 19 of this relay is closed and firing current is supplied to the ignitrons I-1 and I2. This firing current flows through the ignitors 9 of ignitrons I-1 and L2 alternately during opposite half periods of the supply and renders the ignitrons respectively conducting during these half periods. Thus, assume that when the relay RW is first actuated and its contact 19 closed conductor L1 is positive relative to the conductor L2. Initially positive current then flows from conductor L1 through primary P, the lower rectifier 15 associated with ignitron L2, the now closed contact 19 of relay RW, the upper rectifier 33 associated with ignitron 1-1, the ignitor 9 of ignitor L1, the cathode 7 to conductor L2. Ignitron 1-1 is immediately energized and conducts current during the half period through the primary P. Current is then induced in the secondary S and is supplied to weld the work W. During the succeeding half period when conductor L2 is positive relative to conductor L1 current first flows from conductor L2 through the lower rectifier 11 associated with ignitron 1-1, the contact 19 of relay RW, the upper rectifier 17, the ignitor 9 of ignitron 1-2, the cathode 7 of ignitron 1-2, the primary P to conductor L1. Ignitron L2 is now rendered conducting and supplies positive current upward through the primary P. Current thus continues to flow through the primary P and through the work W as the ignitrons 1-1 and I2 are successively rendered conducting during alternate half periods of the supply of conductors Li and L2. Since thyratron ST is rendered conducting early in its positive half periods the relay RW is positively actuated and the current flow through the ignitrons 1-1 and L2 is initiated substantially in synchronism with the pulsations of the supply of the conductors L1 and L2.

Thyratrons ST and AT once rendered conducting continue to conduct for some time after the potential of conductor AL3 becomes negative with respect to the potential of conductor AL2. In addition, thyratrons ST and AT being rendered conducting by the effect of the positive arc drop across thyratron OT conduct when thyratron OT becomes non-conducting. At this instant the potential at the point of junction J1 suddenly becomes negative with respect to the conductor AL2 to which the cathodes of thyratrons ST and AT are connected. But, because the rectifier X1 is connected between junction 11 and conductor AL2 this negative potential is relatively small and does not damage thyratrons ST and AT. If this rectifier were not present the full negative potential of conductor AL1 would, when thyratron OT becomes non-conducting, be suddenly impressed between the control electrodes 31 and and cathodes 29 and 31 of thyratrons ST and AT while the latter are conducting and thyratrons ST and AT would be damaged.

When thyratron AT becomes conducting the charging potential for network WN is reduced and the capacitor 57 in this network discharges. The decaying potential impressed between the control electrode "25 and cathode 23 of the weld thyratron by the network WN has superimposed on it the arc drop across thyratron AT in the same manner as the decaying potential of network SN had impressed on it the arc drop of thyratron OT. At the end of the weld interval the potential impressed by network WN reaches a low magnitude and at the beginning of the half period during which ALI is positive relative to conductor ALZ immediately following the instant when the potential impressed by the network WN becomes small, thyratron WT is rendered conducting. Potential is now impressedthrough the secondary ASl of transformer AT on the capacitor 83 of network AN by the current flow between the second control electrode 33 and the cathode 29 of thyratron ST. This potential is sufficient to render thyratron ST non-conducting.

Relay RW now drops out and the firing potential to the ignitrons Ill and 1-2 is interrupted and which ever of these ignitrons is last to conduct continues to conduct until the end of the corresponding half period and thereafter the current flow through the primary P and work W is interrupted. Thyratron OT, however, remains conducting, relay RS remains energized, solenoid remains energized and the electrode E1 remains in engagement to enable the work W to solidify.

Because thyratron WT is conducting the charging of network l-IN is interrupted. Capacitor 59 of this network now discharges during the hold interval for a time interval adequate to permit the weld nugget or button to harden. At the end of this time interval the potential impressed by the network HN between the control electrode and cathode 23 of thyratron HT reaches a small negative magnitude and at the beginning of the half period when the potential of conductor AL3 is positive relative to the potential of conductor AL2 immediatelyfollowing the decay of the potential of network HN to a low magnitude the hold thyratron HT is rendered conducting.

Charging current is now supplied through the secondary ASZ to the capacitor 53 of the off network ON. Thyratron OT is then rendered non-conducting. Relay RS is de-energized, solenoid O is de-energized and the electrode E1 is retracted from the work W. In addition, the switch SP is opened and during the succeeding half period when the potential of conductor ALl is positive relative to ALZ, the network SN is charged to render thyratron A nonconducting. During the succeeding half period network WN is charged and thyratron WT is rendered non-conducting and this event is in turn followed by the charging of network HN and the rendering non-conducting of thyratron HT. The capacitor of network ON may now discharge during a suitable off interval during which the work W may be moved. If the starting switch FS is now closed or is maintained closed the above-described process is repeated.

Operati0n.-Fig. 1N0n repeat With the switch RNR in the non-repeat position the sequence timer follows the same sequence as with the switch set for repeat until the point at which thyratron HT is rendered conducting. But, now since the contacts i9 and 81 are closed, the charging of the weld network WN is prevented by the conduction of the hold thyratron HT, the anode 21 of which is connected through the contact 49 of switch RNR and through the rectifier iii to the network WN. The thyratron WT then remains conducting so long as the start switch FS is closed. Since the thyratron WT is conducting it prevents the charging of network HN which inturn prevents the blocking of thyratron HT. The capacitor 53 of the network ON then remains charged and thyratron OT remains nonconducting and relay RS de-energized. Thus so long as the switch FS remains closed, the producing of another weld is prevented.

To produce another weld the start switch 'FS must .reinitiates a second welding operation.

Explanation of invention For a more adequate understanding of my invention it may be desirable to explain its basis. This explanation may be presented with the aid of the graphs shown in Fig. 2 in which the various potentials appearing on the electrodes of two thyratrons OT and ST which operate in succession, are plotted as a function of time. In graphs a and b of Fig. 2 time is plotted horizontally and the voltage impressed on the various electrodes vertically. The various curves of Fig. 2 are labeled to indicate the physical parameters which they represent.

The sine curve of graph a represents the potential existing between the buses ALI and ALZ. The mediumcurveof graph a the positive half waves of which coincide with the positive half waves of the sine curve represents the anode potential on thyratron OT when the latter is not conducting with the switch SP closed. The anodecathode potential which is impressed during the negative half periods is the potential drop across the rectifier Xll connected to junction J1 and is represented by the shallow medium curves labeled junction J 1. The corresponding anode-cathode potential on thyratron ST is similarly shown in curve I). It is seen that the curve of graph a is displaced in phase by with respect to the curve of graph b. Again the anode potential, which is impressed during the negative half period, is equivalent to the potential drop across the rectifier X3 connected to the junction J3 and is represented by the loops labeled junction IS. The instants when the power switch and the starting switch PS are closed are represented by the labeled vertical lines. After the switch FS is closed thyratron OT becomes conducting. The instant when the switch FS is closed is shown as occurring very late in the third negative half period. The anode-cathode potential impressed on thyratron OT at this time is positive but is very small so that thyratron OT would not conduct. instead, the thyratron conducts at the beginning of the fourth positive half cycle is indicated by the first heavy arc drop curve of graph a. The conduction hangs over into the succeeding negative half period as also shown by the arc drop curve of graph a. At the instant when the conduction stops the potential of the anode of thyratron OT drops sharply to a negative magnitude but the magnitude is the small negative magnitude at junction II.

The potential impressed between the control electrode and cathode of thyratron ST is represented by the heavy broken-line curve of graph b. This curve shows that the potential is at a substantial negative magnitude until the switch FS is closed. Thereafter it rises gradually to Zero magnitude. The potentials of the arc drop across thyratron OT are as represented by the square wave sections of the decay curve. The critical potential which is assurned to be zero is exceeded at the beginning of the second positive half cycle from the end, and at this point thyratron ST becomes conducting as shown. As can be seen, the control potential drops to a negative magnitude at the instant when the thyratron OT becomes nonconductive but this magnitude is small and does not damage thyratron ST.

The relationship shown in Fig. 2 between the potentials impressed on thyratrons OT and ST is the same as the relationship impressed on the electrodes of any two of the thyratrons which operate in succession. The operation of the apparatus should thus be understood from Fig. 2 in connection with the above explanation.

l1 Descriptin.Figure 3 The apparatus shown in Fig. 3 is similar to the apparatus shown in Fig. 1 except that it is conceived for use with a so-called two-stage starting switch or foot switch (PS1). Such a switch has two contacts (111 and 113) which close in succession. The first contact (111) is connected to open the valve V which supplies the fluid for moving the movable electrode E1 but not to start the welding process. The second contact (113) which closes after the first contact (111) is closed is connected to start and complete the welding process. The purpose of the first contact is to enable the operator to actuate the movable electrode El so that the work may be properly centered with respect to the electrodes E1 and E2.

The apparatus shown in Fig. 3 specifically differs from the apparatus shown in Fig. 1 only in the portion of the circuit including the thyratron OT. The portion of the circuit including the other thyratrons is the same as that shown in Fig. 1 and are not shown in Fig. 3.

The apparatus shown in Fig. 3 includes the twostage starting switch PS1, and auxiliary relay AR in addition to components similar to that included in the Fig. 1 apparatus. The auxiliary relay AR has a normally open contact 114 between the anode 21 of thyratron OT and the pressure switch SP and this is the only available connection between these points. Thus the welding process cannot go forward until the contact 114 is closed. The start switch has two contacts 111 and 113 which are adapted to close in succession. The first of these 111 directly closes the anode circuit of the thyratron OT to energize directly the coil of the starting relay RS1. One of the contacts 115 of this relay is connected to energize the solenoid 0. Thus, the closing of the first contact 111 of PS1 causes the valve V to open and the electrode El to be moved toward the work W so that the electrode may be centered. The second contact 113 which closes later is connected in circuit with the coil of auxiliary relay AR. When this contact closes relay AR is energized and the cathode of the thyratron OT is connected to the conductor AL2 through this contact 113 and through a normally open contact 117 of auxiliary relay AR. At the same time the cathode of thyratron OT is connected to bus AL2 through the now closed contacts 119 of relay RS1 and 291 of relay AR. This locks thyratron OT and relay AR in so that even if the operating switch PS1 is now released, the welding sequence will go forward to completion. In addition, the contact 114 also closes so that the welding process may go forward.

0perati0n.-Figure 3 Before the operation of the system shown in Fig. 3 is initiated the movable electrode is centered with respect to the work W, as has been explained by closing only the contact 111 of the switch PS1 and setting the electrode El properly. Thereafter the second contact 113 is closed energizing relay AR. At its lower now closed contact 117 relay AR closes a circuit to the conductor AL2 from the cathode of the thyratron OT through contact 113 of the switch. At its upper now closed contact tie anode 21 of the thyratron OT is connected to the still open terminals of the pressure switch SP.

By the closing of the start switch PS1 the starting rela RS1 is actuated causing the solenoid O to be energized and the electrode to move into engagement with the work W. in addition, the thyratron OT is locked in through the lower now closed contact E9 of the starting relay RS1 in series with the lower contact 201 of relay AR. When adequate pressure is built up on the work the pressure switch SP is closed and the welding operation proceeds as described above.

Descripti0n.Figure 4 The apparatus shown in Fig. 4 dilters from the apparatus shown in Fig. 1 only with respect to the manner in which the OE network ON is charged from the hold thyratron HT. In the system shown in Fig. 4 the ofi network is charged through a tertiary winding 282 of the supply transformer T2. This winding is connected at one terminal to the anode 21 of the hold thyratron HT through conductor AL4 and a rectifier 121 poled to conduct current in the direction from the terminal to the anode. At the other terminal the winding 282 is connected to the oil network ON. The circuit is completed through the conductor AL2 to which both the cathode 23 of the hold thyratron HT and the oil network ON are connected Operation-Figure 4 The system shown in Fig. 4 operates the same as the system shown in Fig. 1 as far as the thyratrons OT, ST, AT and WT and their associated networks are concerned. When the thyratron WT is rendered conducting to stop the supply of welding current the hold network HN times out. At the end of the hold interval the hold thyratron HT is rendered conducting. This thyratron is supplied from the winding 282 and causes the capacitor 53 in the ofi network ON to be charged to de-energize the oil? thyratron OT and complete the welding process.

The system shown in Fig. 4 has the advantage that the potential to which the capacitor of the off network ON is charged is more consistently uniform than in the system shown in Fig. 1 in which the potential of the ctl capacitor may vary because of the poor regulation of the transformer ATZ and its saturation properties. in the system shown in Fig. 4 it is essential that the rectifier which is connected between the junction l3 and the anode of thyratron HT when the repeat nonrepeat switch RNR is set for non-repeat must have a very high back re sistance. if this back resistance is low, current would flow from the thyratron AT when it is rendered conducting to charge the off network ON in the non-repeat position of the switch and the operation would thus be terminated before it could start.

Conclusion The apparatus shown herein is a Sequence Timer for an industrial process which has a minimum number of tubes and associated components. Such a circuit a relatively low cost and its maintenance cost is also low.

Thus for precision welding, the relay RW is replaced by the input circuit to a Heat Control Unit such as is shown in an application to Hubert W. Van Ness and Edward C. Hartwig, filed concurrently herewith and assigned to Westinghouse Electric Corporation, Patent No. 2,728,031 issued December 20, 1955. In such a situation the squeeze thyratron ST would supply a network such as the input network HCN disclosed in the above mentioned application in the same manner as it is supplied by the thyratron HT.

While I have shown and described certain specific embodiments of my invention, many modifications thereof are possible. My invention, therefore, is not to be limited except as insofar necessitated by the spirit of the prior art.

I claim as my invention:

1. In combination a first conductor, a second conductor, a third conductor, means for impressing a first alternating potential between said first conductor and said second conductor, means for impressing a second alternating potential between said third conductor and said second conductor, said potentials being so related that the potential of said first conductor is substantially in opposite phase with respect to the potential of said third conductor, a first electric discharge device having an anode, a cathode and a control electrode, a second electric discharge device having an anode, a cathode and a control electrode, said second device being of the gaseous type which cleans up if a negative potential of substantial magnitude is impressed between its control electrode and its cathode while it is conducting, inductive reactance means, means including said inductive reactance means for connecting said anode and cathodeof said first device between said first conductor and said second conductor respectively, a time constant network, an impedance, means for connecting said impedance between said first conductor and said network, means for connecting said network to said control electrode of said second device, means for connecting said anode and cathode of said second device between said third conductor and said second condue-tor respectively, means for connecting the junction of said impedance and said network to said anode of said first device, and rectifier means connected between said second conductor and said anode of said first device in a direction to conduct from said second conductor to said anode.

2. In combination a first conductor, a second conductor, a third conductor, means for impressing potentials between said conductors such that the potential of said second conductor is at every instant intermediate the potentials of said first and third conductors respectively, a first electric discharge device of low impedance having an anode and a cathode, a second electric discharge device having an anode, a cathode and a control electrode, said second device being of the gaseous type which cleans up if a negative potential of substantial magnitude is impressed between its control electrode and its cathode while it is conducting, inductive reactance means, means including said inductive reactance means for connecting said anode and cathode of said first device between said first and second conductors respectively, means for con-- necting said anode and cathode of said second device be tween said third and second conductors respectively, impedance means, rectifier means, means for connecting said impedance means and said rectifier means in series between said first and second conductors, said rectifier means being connected, to conduct positive current from said second conductor to said first conductor, means for connecting said anode of said first device to the electrical junction of said impedance means and said rectifier means, and means for connecting said control electrode to said junction.

3. In combination a first conductor, a second conductor, means connected, to said first and second conductors for impressing an alternating potential between said first conductor and said second conductor, an electric discharge device having an anode and a cathode, a symmet: rically conductive impedance means, symmetrically conductive means connectedto said first conductor and said anode for connectingsaid impedance means between said first conductor and said anode, first rectifier means, means connected to said second conductor and said anode for connecting said first rectifier means between said second conductor and said anode, said first rectifier means being connected to conduct positive current from said second conductor to said anode and having a forward impedance which is low compared to the impedance of said impedance means so that when said second conductor is electrically positive relative to said first conductor the potential drop between said second conductor and said anode is low compared to the potential drop between said anode and first conductor, inductive reactance means, sec ond rectifier means, and means connected to said anode, cathode, first conductor and second conductor for connecting in aseries circuit in current conducting relationship saidfirst conductor, said second conductor, said inductivereactance means, said second rectifier means, said anode and said cathode, said second rectifier means being connected in said series circuit between said first conductor and said anode to conduct positive current from said firs-t conductor to said anode.

4. In combination a first conductor, a second conductor, means connected to said first and second conductors for impressing an alternating potential between said first till conductor and said second conductors, an electric discharge device having an anode and a cathode, symmetrically conductive impedance means, symmetrically conductive means connected to said first conductor and said anode for connecting said impedance means between said first conductor and said anode, first rectifier means, means connected to said second conductor and said anode for connecting said first rectifier means between said second conductor and said anode, said first rectifier means being connected to conduct positive current from said second conductor to said anode and having a forward impedance which is low compared to the impedance of said impedance means so that when said second conductor is electically positiverelative to said first conductor the potential drop between said second conductor and said anode is low compared to the potential drop between said anode and first conductor, load means, second rectifier means, and means connected to said anode, cathode and first conductor for connecting in a series circuit in conducting relationship said first conductor, said second conductor, said load means, said second rectifier means, said anode and said cathode, said second rectifier means being connected in said series circuit between said first conduetor and said anode to conduct positive current from said first conductor to said anode.

5. In combination a first conductor, a second conductor, a third conductor, means for impressing alternating potentials between said conductors such that said second conductor is at a potential intermediate the potentials of said iii-stand third conductors, a first plurality of electric discharge devices each having an anode, a cathode and a control electrode, a second plurality of electric discharge devices each having an anode, a cathode and a control electrode, inductive reactance means associated with each of said devices, atime constant network associated with each of said devices, means including the associated inductive reactance'mean-s for connecting the anode and cathode of each device of said first plurality between said first conductor and said second conductor respectively, means including the associated inductive reactance means for connecting the anode and cathode of each device of said second plurality between said third conductor and said second conductor respectively, and means for connecting each network in charging relationship between the control electrode of its associated device and the anode of a device of the other plurality.

6. In combination a first conductor, a second conductor, a third conductor, means for impressing alternating potentials between said conductors such that said second conductor is at a potential intermediate the potentials of said first and third conductors, a first plurality of electric discharge devices each having an anode, a cathode and a control electrode, a second plurality of electric discharge devices each having an anode, a cathode and a control electrode, each of said devices being of the gaseous type which cleans up if a negative potential of substantial magnitude is impressed between its control electrode and cathode while it is conducting, inductive re'actance means associated with each of said devices, a time constant not work associated with each of said devices, impedance means associated with each of said devices, rectifier means associated with each of said devices, means including the associated inductive reactance means for connecting the anode and cathode of each device of said first plurality between said first conductor and said second conductor respectively, means including the associated inductive reactance means for connecting the anode and cathode of each device of said second plurality between said third conductor and said second conductor respectively, means for connecting each impedance and the corresponding rectifier means in series between said second conductor and that other conductor to which the anode of the corresponding discharge device is connected and means for connecting each time constant network between the control electrode of the associated device and the difi'erent junc- 15 tion of impedance means and rectifier means associated with a device of the other plurality.

7. In combination a first conductor, a second conductor, a third conductor, means connected to said conductors for impressing potentials therebetween such that said second conductor is at every instant at a potential intermediate said first conductor and said third conductor, a fourth conductor, means connected to said fourth conductor and said second conductor for impressing a potential between said fourth conductor and said second conductor, first, second, third and fourth electric discharge devices, each device having an anode, a cathode and a control electrode, first, second, third, and fourth time constant networks, first and second connecting means connected to said first conductor, said second conductor and said first and third devices respectively for connecting respectively said anodes and cathodes of said first and third devices in current conducting relationship between said first and second conductors to conduct positive current from said first conductor to said second conductor, said first and second connecting means each including inductive reactance means, third connecting means con necting said anode and cathode of said second device in current conducting relationship between said third conductor and said second conductor to conduct positive current from said third conductor to said second conductor, said third connecting means including inductive reactance means, fourth connecting means connecting said anode and cathode of said fourth device in current conducting relationship between said fourth conductor and said second conductor to conduct positive current from said fourth conductor to said second conductor, said fourth connecting means including said fourth network whereby said fourth network is charged by the current conducted through said fourth device, means connecting said first network in charging relationship between said first conductor and said control electrode of said second device to charge said first network by positive current flow between said first conductor and said control electrode of said second device when said first device is non conducting, means connecting said second network in charging relationship between said third conductor and said control electrode of said third device to charge said second network by positive current flow between said third conductor and said control electrode of said third device when said second device is non-conducting, means connecting said third network in charging relationship between said first conductor and the control electrode of said fourth device to charge said third network by positive current flow between said first conductor and said control electrode of said fourth device when said third device is non-conducting, and means connecting said fourth network in blocking relationship between the control electrode and cathode of the first device.

8. In combination a first conductor, a second conductor, a third conductor, a first electric discharge device having an anode, a cathode and a control electrode, a second electric discharge device having an anode, a cathode and a control electrode, said second device being of the gaseous type which cleans up if a negative potential of substantial magnitude is impressed between its control electrode and its cathode while it is conducting, inductive reactance means, means including said inductive reactance means for connecting said anode and cathode of said first device between said first conductor and said second conductor respectively, a time constant network, an im pedance, means for connecting saidimpedance between said first conductor and said network, means for connecting said network to said control electrode of said second device, means for connecting said anode and cathode of said second device between said third conductor and said second conductor respectively, means for connecting the junction of said impedance and said network to said anode of said first device, and rectifier means connected between said second conductor and said anode of said 15 first device in a direction to conduct positive current from said second conductor to said anode.

9. In combination a first conductor, a second conductor, a third conductor, means connected to said conductors for impressing potentials therebetween such that the potential of said second conductor is at every instant intermediate the potentials of said first and third conductors, first, second, third and fourth electric discharge devices, each of said devices having an anode, a cathode, and a control electrode, first, second, third and fourth time constant networks, first and second means respectively connecting in current conducting relationship the anodes of said first and third devices to said first conductor, third and fourth means respectively connecting in current conducting relationship the anodes of said second and fourth evices to said third conductor, fifth and sixth means connected to said cathodes of said first and third devices for respectively connecting in current conducting relationship said cathodes of said first and third devices to said second conductor, seventh and eighth means respectively connecting said cathodes of said second and fourth devices to said second conductor, means connected to the anode of the first device, to the first network and to the control electrode of the second device for connecting said first network in charging relationship between the anode of the first device and the control electrode of the second device, means connecting said second network in charging relationship between the anode of the second device and the control electrode of the third device, means connecting said third network in charging relationship between the anode of the third device and the control electrode of the fourth device, and means connecting said fourth network in charging relationship between the anode of the fourth device and the control electrode of the first device.

10. In combination a first conductor, a second conductor, a third conductor, means connected to said condoctors for impressing potentials therebetween such that the potential of said second conductor is at every instant intermediate the potentials of said first and third conductors, first, second, third and fourth electric discharge devices, each of said devices having an anode, a cathode, and a control electrode, first, second, third and fourth time constant networks, first and second means respectively connected to the anodes and cathodes of said first and third devices and to said first and second conductors for connecting respectively said anodes and cathodes of said first and third devices in current conducting relationship between said first conductor and said second conductor to conduct positive current from said first conductor to said second conductor, said first and second means each including inductive reactance means, third and fourth means respectively connecting in current conducting relationship said anodes and cathodes of said second and fourth devices between said third and second conductors to conduct positive current from said third to said second conductor, said third and fourth means each including inductive reactance means, means connected to the anode of the first device to the first network and the control electrode of the second device for connecting said first network in charging relationship between the anode of the first device and the control electrode of the second device, means connecting said second network in charging relationship between the anode of the second device and the control electrode of the third device, means connecting said third network in charging relationship between the anode of the third device and the control electrode of the fourth device, and means connecting said fourth network in charging relationship between th anode of the fourth device and the control electrode of the first device.

11. In combination a first conductor, a second conductor, a third conductor, means connected to said conductors for impressing potentials therebetween such that the potential of said second conductor is at every instant intermediate the potentials of said first and third conductors, first, second, third and fourth electric discharge devices, each of said devices having an anode, a cathode, and 'a control electrode, first, second, third and fourth time constant networks, first and second means respectively connected to the anodes and cathodes of said first and third devices and to said first and second conductors for connecting respectively said anodes and cathodes of said first and third devices in current conducting relationship between said first conductor and said second conductor to conduct positive current from said first conductor to said second conductor, said first and second means each including inductive reactance means, third and fourth means respectively connecting in current conducting relationship said anodes and cathodes of said second and fourth devices between said third and second conductors to conduct positive current from said third to said second conductor, said third and fourth means each including inductive reactance means, means connected to the anode of the first device, to the first network and to the control electrode of the second device for connecting said first network in charging relationship between the anode of the first device and the control electrode of the second device, said first network to be charged by the flow of positive current from said anode of said first device to said control electrode of said second device when said first device is non-conducting, means connecting said second network in charging relationship between the anode of the second device and the control electrode of the third device, said second network to be charged by the dew of positive current from said anode of said second device to said control electrode of said third device when said second device is non-conducting, means connecting said third network in charging relationship between the anode of the third device and the control electrode of the fourth device, said third network to be charged by the flow of positive current from said anode of said second device to said control electrode of said third device when said third device is non-conducting, and means connecting said fourth network in charging relationship between the anode of the fourth device and the control electrode of the first device, said last named connected means including a transformer having a primary connected in circuit with said anode and cathode of said fourth device and a secondary connected in circuit with said fourth network and said control electrode and cathode of said fourth device to conduct positive current from said network to said control electrode to charge said fourth network when said fourth device is conducting.

References Cited in the file of this patent UNITED STATES PATENTS 2,210,523 Blumlein Aug. 6, 1940 2,298,240 Toepfer Oct. 6, 1942 2,403,955 Schlesinger July 16, 1946 2,424,999 Ostlund et a1. Aug. 5, 1947 2,533,369 Hartwig Dec. 12, 1950 2,574,373 Bivins Nov. 6, 1951 2,586,592 Alcorn Feb. 19, 1952 2,594,016 Hartwig Apr. 22, 1952 2,611,863 Bivins Sept. 23, 1952 2,614,240 Bivins Oct. 14, 1952 2,659,008 Floyd Nov. 10, 1953 2,677,052 Rockafellow Apr. 27, 1954 

